Filters away from an input semantics any literals whose realisation is
strictly morphological. The first argument tells us helps identify the
morphological literals -- it associates literals with morphological stuff;
if it returns Nothing, then it is non-morphological

attachMorphmorphfn sem cands does the bulk of the morphological
input processing. We use morphfn to determine which literals in
sem contain morphological information and what information they contain.
Then we attach this morphological information to the relevant trees in
cand. A tree is considered relevant w.r.t to a morphological
literal if its semantics contains at least one literal whose first index
is the same as the first index of the morphological literal.